Q.Mandarin Orange Leaf Problem
Hi- my miniature mandarin orange doesn’t produce fruit. The underside of many leaves has a disgusting looking white and black substance clinging to it. Almost looks like white crystals. Is this Leaf Miner? Please tell me I can blast it with something without destroying the pest’s natural enemies!

Certified GKH Gardening Expert
It doesn't sound like leaf miners but it could be mealy bugs or scale. The black is sooty mold. Here is more:
https://www.gardeningknowhow.com/edible/fruits/tangerine/mandarin-orange-tree-care.htm
https://www.gardeningknowhow.com/edible/fruits/oranges/no-fruit-on-orange-trees.htm
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=29424
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm